This invention relates to a method of removing individual ones of a mixture of gases within a controlled environment container and to apparatus for selectively adsorbing components of a gas mixture.
Adsorbent materials can be used to change the composition of a mixture of gases by selective adsorption, using a material which has different tendencies to adsorb different components of the mixture. For example, nitrogen can be generated by selective adsorption of oxygen from air, -or by adsorbing nitrogen and then desorbing the adsorbed gas for collection.
It can be desirable to adjust the content in a mixture of gases of two or more of the components. For example, the storage life of perishable goods within a transportation container can be affected by the amounts of oxygen and of carbon dioxide in the environment within the container.
It is known to use twin adsorbent chambers to separate components of a gas mixture. The adsorbent in the chambers adsorbs one of the components of the mixture preferentially relative to another of the components. Apparatus which comprises two adsorbent chambers allows the adsorbent in one of the chambers to be used for separation while the adsorbent in the other of the chambers is regenerated by passage through it of a purge stream of gas supplied from the outlet of the chamber that is in use for separation.
The present invention provides a technique for separating components of a mixture of gases in which the purge gas is supplied from the chamber that is in use for separation to the chamber that is to be regenerated in a pulse.
Accordingly, in one aspect, the invention provides a method of removing individual gas components of a mixture of gases within a controlled environment container using apparatus which comprises first and second chambers which are arranged in parallel for the gas mixture to flow through them from their inlet ends to their outlet ends and which contain adsorbent material with different affinities for the gas components of the mixture, the method comprising:
(a) supplying the gas mixture to the first chamber so as to pressurise it, while allowing a purge stream to flow from the outlet end of the first chamber to the second chamber to purge adsorbed gas from the adsorbent material in the second chamber, and
(b) supplying a purge pulse of gas from the outlet end of the first chamber to the second chamber to purge adsorbed gas from the adsorbent material in the second chamber, the rate of flow of the purge stream being less than that of the purge pulse.
The method of the invention has the advantage that, by use of a purge pulse of gas from the first container to the second container, the efficiency which the components of the gas mixture can be separated can be increased. This can arise because of the use of the pulse of gas mixture which contains a relatively high proportion of the gas components for which the affinity of the adsorbent is least strong from the first chamber to purge adsorbed gas from the second chamber.
The purge stream and purge pulse will generally flow between the chambers through purge stream and purge pulse conduits, which may be separate or may be provided by a common conduit. Generally, at least one of the purge stream and the purge pulse, and preferably both, will be supplied to the second chamber at the outlet end thereof so that the flows of gas for adsorption and purge gas are countercurrent. When a common conduit is used, its capacity can be controlled by means of a valve which can be removed between first and second positions in which it accommodates the purge stream and the purge pulse respectively. Preferably, the ratio of the rate of flow of gas (measured in terms of weight of gas) in the purge pulse to that in the pulse stream is at least about 1.5, more preferably at least about 1.8, especially at least about 2.0. When the relative flow rates are achieved by different flow capacities in the purge stream and purge pulse conduits, it can be appropriate for the ratio of the capacity (volumetric) of the purge pulse conduit to that of the purge stream conduit to be at least about 1.5, preferably at least about 1.8, especially at least about 2.0. The ratio will generally be less than about 4.0, for example less than about 3.0.
The purge stream can flow from the outlet end of the first chamber to the second chamber substantially continuously while the pressure in the first chamber is greater than that in the second chamber. For example, when the purge stream flows through a purge stream conduit which connects the first and second chambers, it can contain an orifice valve. When a common conduit is used for the purge stream and the purge pulse, it might contain an orifice valve to control the rate of flow of the purge stream; that orifice valve might be capable of being opened to accommodate the flow of the purge pulse, or a by-pass section might be opened for the purge pulse.
Purged gas is discharged from the second chamber while one or both of the purge pulse and the purge stream is supplied to the second chamber. When the purge pulse or the purge stream are supplied at the outlet end of the second chamber, the purged gas will be discharged from the inlet end. The purged gas that is discharged from the second chamber can be vented to the controlled environment container from which the gas mixture is supplied to the apparatus. The invention can thus be used to increase the amount of the component of the gas mixture in the controlled environment container that is the dominant component in the purged gas. The component might be vented to another vessel when the method of the invention is used to purify that component. Alternatively or in addition, the purged gas that is discharged from the second chamber can be vented to atmosphere. A combination of venting to atmosphere and venting to the controlled atmosphere container can be appropriate when the identify of the dominant component of the purged gas changes as it flows.
Preferably, the method of the invention includes the step of venting the first chamber to discharge preferentially the component of the gas mixture for which the affinity of the adsorbent is least strong. The venting step will generally take place after the step of supplying a purge pulse of gas to the second chamber. The vented gas can be discharged to atmosphere when the method is used to reduce the amount of that component of the gas mixture in the controlled atmosphere container. The vented gas can be discharged to the container when the method is used to increase the amount of that component of the gas mixture in the controlled atmosphere container. The vented gas might be discharged to a vessel for storage when the method of the invention is used to purify that gas for collection. The technique provided by the present invention involving use of a pulse or purge gas has the advantage that it enhances the purity with which the least strongly adsorbed component or components of the gas mixture can be separated from the mixture. The invention thus provides an effective technique for purifying that component of the mixture.
The use of first and second chambers containing adsorbent allows the two quantities of adsorbent in the two chambers to be used for adsorption alternately, with one of the quantities being purged for reuse while the other is being used for adsorption. Preferably, the nature and configuration of the first chamber are substantially the same as those of the second chamber. Preferably, the adsorbent material contained within the first chamber is substantially the same as that within the second chamber. The method of the invention preferably includes the steps of equalising the pressures in the chambers and repeating the steps (a) to (d) referred to above with the respective functions of the first and second chambers reversed.
The duration of the steps of the method will be controlled according to factors including the relative proportions of the components in the gas mixture, the capacity of the adsorbent material in the chambers to adsorb selected ones of the components. For example, it can be preferred for the method to include a step of controlling the period for which the first chamber is vented to atmosphere to optimise venting of the component of the gas mixture for which the affinity of the adsorbent is least strong and retention by the adsorbent of the component of the gas mixture for which the affinity of the adsorbent is most strong. This can allow effective separation of the components of the gas mixture.
The adsorbent in the chambers can comprise a single material. It can be appropriate for there to be more than one adsorbent material in the chamber. The selection of adsorbent material will be made according to the nature of the components in the gas mixture and the adjustments that are required to the composition of the mixture.
Examples of suitable adsorbent materials which might be used in the present invention include activated alumina, silica based materials, zeolites and carbon based materials. Molecular sieve adsorbents will be appropriate for many applications. The high pressure oxygen adsorbent molecular sieve sold by UOP under the trade mark PSA O2HP has been found to be particularly suitable for controlling the amounts of oxygen and carbon dioxide in a gas mixture consisting largely of nitrogen in addition to those gases. Adsorbent materials which have desiccant properties can be included in the chamber (with other adsorbent materials if appropriate) when the gas mixture includes moisture which should be removed. An example of a suitable desiccant material is an activated aluminium oxide.
In another aspect, the invention provides a twin bed adsorption apparatus for selectively adsorbing components of a gas mixture, which comprises first and second chambers arranged in parallel for the gas mixture to flow through them from their inlet ends to their outlet ends and which contain adsorbent material with different affinities for the gas components of the mixture, each of the chambers having (i) an inlet port for the gas mixture at its inlet end, (ii) an outlet port at its outlet end for discharge of the component of the gas mixture for which the affinity of the adsorbent is least strong, (iii) a purge outlet port for gas that is supplied to the chamber to purge gas adsorbed by the adsorbent, the apparatus including:
(a) a purge stream conduit for providing a purge stream of gas between the chambers when their internal pressures are different, from the outlet end of the chamber with a higher internal pressure to the chamber with a lower internal pressure, and
(b) a purge pulse conduit for providing a purge pulse of gas from the outlet end of the chamber with a higher internal pressure to the chamber with a lower internal pressure,
the capacity of the purge pulse conduit being significantly greater than the capacity of the purge stream conduit.
Preferably, the purge outlet ports are provided at the inlet ends of the chambers. Preferably, they are arranged so that the purge gas from the chambers can be directed to atmosphere or to a container from which the gas mixture is supplied to the apparatus.
Preferably, the apparatus includes a pressure equalisation conduit by which the chambers are connected, for example at their outlet ends, the pressure equalisation conduit including a two way valve by which the conduit can be opened for rapid equalisation of pressures in the chambers. During equalisation of the pressures in the chambers, the purge pulse conduit can be opened to increase the speed of the equalisation.
The technique of the invention can also involve the removal of components from a gas mixture by degradation. For example, the method of the invention can include a step of exposing the gas mixture to a treatment step which causes one or more components of the mixture to degrade. The step might involve for example the application of heat or pressure, or exposure to irradiation such as an electron beam or ultraviolet radiation. Exposure to ultraviolet radiation can be used to degrade hydrocarbon components of a gas mixture, especially ethylene. The apparatus of the invention can include an appropriate ultraviolet radiation source. Preferably, the apparatus includes appropriate heat exchanger components to allow waste heat from a compressor that is used to move gas within the controlled environment container to heat the gas mixture to be degraded. It is particularly preferred that the temperature of gas to be degraded by exposure to ultraviolet radiation be increased to enhance the degradation.
The adsorption technique provided by the present invention can be used to control the atmosphere in a container which might be used for transportation or storage or both of vulnerable materials. An example of an application might be for transportation and/or storage of perishable foodstuffs. The container might be transported by land (for example by road or by rail), sea or air. The invention is applicable to containers with a capacity of at least about 1000 litres, for example at least about 104 litres, especially at least about 3xc3x97104 litres.